Double Sided Si(Ge)/Sapphire/III-Nitride Hybrid Structures

Several of the patented methods included in this suite of technologies enable super-hetero-epitaxy of rhombohedral/cubic compound semiconductors on specially oriented trigonal (e.g. sapphire) or hexagonal (e.g. quartz) crystal wafer substrates. This includes alignment of the growth crystal lattice with the underlying substrate lattice to minimize misfit strain-induced dislocation defects in the growing crystal. Thus thicker, defect-free crystal layers can be made. Rhombohedral/Cubic crystal twin defects which is 60 degree rotated on [111] orientation in a rhombohedral/cubic SiGe layer structure can be reduced to well less than 1% by volume, essentially providing a defect-free semiconductor material. Alternately, engineered lattice structures with a high degree of twinning can provide SiGe with improved thermoelectric properties due to the phonon scattering that inhibits thermal conduction without compromising electrical conductivity. Additional patented technologies in this suite provide for physical vapor deposition (PVD) growth methods utilizing molten sputtering targets and thermal control of heated substrates, including electron beam heating, in order to give the atoms in the sputtered vapor or on the substrate surface the energy needed for the desired crystal growth. The remaining patented technologies enable x-ray diffraction methods for detecting and mapping crystal twin defects across the entire as-grown semiconductor layer. These defects are critical to the performance of any semiconductor device manufactured from such compound semiconductor materials.

This innovation is based on a new fabrication method that alleviates the thermal loading requirement of the substrate, which previously required surface temperatures within the range of 850 to 900C. Our method employs a new thermal loading requirement of sapphire substrate for growing single crystal SiGe on sapphire substrate, in the range of 450 to 500C. SiGe/sapphire wafers produced via this process show a high reflectivity without the discoloration that appears in low quality films.

Performance of solar cells and other electronic devices such as transistors can be improved greatly if carrier mobility is increased. Si and Ge have Type-II bandgap alignment in cubically strained and relaxed layers. Quantum well and super lattice with Si, Ge, and SiGe have been good noble structures to build high electron mobility layer and high hole mobility layers. However, the atomic lattice constant of Ge is bigger than that of Si and direct epitaxial growth generates large density of misfit dislocations which decrease carrier mobility and shorten device life time. So it required special buffer layers such as super lattice or gradient indexed layers to grow Ge on Si wafers or Si on Ge wafers. The growth of these buffer layers takes extra effort and time such as post-annealing process to remove dislocations by dislocation gliding inside buffer layer. This invention is a fabrication method for high mobility layer structures of rhombohedrally aligned SiGe on a trigonal substrate. The invention utilizes C-plane (0001) Sapphire which has a triangle plane, and a Si (Ge) (C) (111) crystal or an alloy of group TV semiconductor (111) crystal grown on the Sapphire.

This process utilizes an electron beam flood gun to irradiate the sapphire surface as a means of raising the surface temperature. As the electrons collide with the top layers of surface atoms, the substrate absorbs much of the energy through thermalization losses from the electrons. Subsequently, the surface temperature rises and this modifies the atomic surface structure into a form conducive to single crystal SiGe epitaxy. Moreover, while tested on sapphire, this method applies to any other wafer material, providing a broad new means of changing surface temperature and atomic structure independently of the substrate heater.

This innovation is based on two new X-Ray Diffraction measurement methods for integral detection and spatial wafer mapping of twin defects in rhombohedrally aligned cubic semiconductor epitaxial layers. By using this innovation as a quality monitoring and control technique, epitaxial growth methods can be optimized to reduce twin defects commonly observed in the new rhombohedrally grown cubic semiconductors on trigonal crystal substrates. The technology was developed to support related NASA inventions for epitaxial growth of rhombohedrally aligned cubic semiconductors, as described in LAR-16868, LAR- 16872, and LAR-17185 (covered under two patent applications in process, including US20070222034 and US Patent #7341883). Other patent applications are anticipated. The technology was also reported in Rhombohedral Epitaxy of Cubic SiGe on Trigonal c-Plane of Sapphire, Journal of Crystal Growth 310 (2008) 27242731.